To get every dragon you have to complete every mission for the virtwyn, chrysalis, and pariah. Make sure to try to complete every single objective. If you fail the mission to win the primary objective or any other objective com back to that mission with a stronger dragon and get the rest of the objectives.
Get All Creatures
Go to the main menu and press Left, Right, Left, Left, Right, X
Get Invincibility
Go to the main menu and press Left, Left, Right, Right, Up, Up, X
Get Crystal
Go to the main menu and press Left, Right, Left, Up, Right, Down, Left, X, X
Get All Multi Player Levels
Go to the main menu and press Left, Right, Left, Right, Up, Left, Right, X
Get All Objectives Completed
Go to the main menu and press Left, Right, Right, Down, Up, Down, Down, X
